Method for the treating in exhaust gas and exhaust gas treating system

ABSTRACT

The present invention provides a method for the treatment of mercury present in exhaust gas wherein, after a chlorinating agent is added to exhaust gas containing nitrogen oxides, sulfur oxides and mercury, the exhaust gas is subjected to a reductive denitration treatment in the presence of a solid catalyst and then to wet desulfurization using an alkaline absorbing fluid, the method being characterized by measuring the mercury concentration in the exhaust gas after the wet desulfurization; calculating a predicted value of the inlet mercury concentration before the reductive denitration treatment on the basis of the measured mercury concentration; and controlling the feed rate of the chlorinating agent added prior to the reductive denitration treatment, according to the deviation of the predicted value from a reference inlet mercury concentration, as well as a system for the treatment of exhaust gas. Thus, the present invention provides an exhaust gas treatment process permitting the removal of mercury, and this process makes it possible to achieve efficient operation of the system and maintain the performance thereof without exerting any adverse influence the units within the system.

TECHNICAL FIELD

[0001] This invention relates to an exhaust gas treatment process and,in particular, a mercury removal method employed in an exhaust gastreatment process. More particularly, it relates to a method forremoving metallic mercury effectively from exhaust gas in a system fordesulfurizing a large volume of exhaust gas.

BACKGROUND ART

[0002] Harmful trace components such as mercury are present in exhaustgas resulting from the combustion of coal or heavy oil, and it isgenerally difficult to remove them in the existing exhaust gas treatmentsystems. It is believed that mercury exists in exhaust gas chiefly asmetallic mercury (Hg) or mercury chloride (HgCl₂). Since HgCl₂ is easilyabsorbed into water, it can be removed in a desulfurizing absorptiontower or the like. However, metallic mercury (Hg) has a very lowsolubility in water and cannot be absorbed in a desulfurizing absorptiontower. Consequently, there is the possibility that metallic mercuryvapor may be discharged from the stack.

[0003] For this reason, an activated carbon adsorption method, a sodiumhypochlorite absorption method and the like have conventionally beenemployed as Hg removal techniques.

[0004] For the activated carbon adsorption method, a method in whichactivated carbon powder is blown into exhaust gas and recovered with abag filter has already been put to practical use. However, this methodis employed chiefly for the treatment of exhaust gas from refuseincineration, and its application to a large-volume gas such as exhaustgas from an electric power plant is not known.

[0005] For the sodium hypochlorite absorption method, there is a knownmethod where an additive such as sodium hypochlorite is directly added,for example, to the cooling water of the cooling tower, the absorbingfluid of the desulfurizing absorption tower, or the feed water orcirculating water of the wet dust collector. However, in all cases, anadditive is added to a main unit in an exhaust gas treatment plant, andsome additives involve the risk of interfering with its intrinsicfunction. For example, it is conceivable that the cooling water has alow pH and hence requires a large amount of an oxidizing agent, formingperoxides in the cooling tower, and oxidizing sulfurous acid in the wetdust collector to cause an increase in acidity. Moreover, this methodhas been adapted chiefly to the treatment of exhaust gas from refuseincineration, and is not suitable for the treatment of a large-volumegas such as exhaust gas from an electric power plant.

[0006] Meanwhile, metallic mercury is hardly soluble in water and hencepasses through the desulfurizer, as described above. If metallic mercurycan be made soluble in water, it may be removed in the desulfurizer.Accordingly, it is conceivable that, if metallic mercury can beconverted into water-soluble mercury chloride on the catalyst of thedenitrator, the mercury chloride may be removed in the desulfurizerinstalled downstream thereof. That is, an exhaust gas treatment methodin which a chlorinating agent (e.g., hydrogen chloride) for convertingmetallic mercury into mercury chloride is injected on the upstream sideof the denitrator is believed to be effective.

[0007] However, the addition of an excessive amount of the chlorinatingagent involves a problem in that it can cause corrosion of the flue anddownstream units of the system and eventually shortens the life of theplant equipment. Moreover, if the chlorinating agent is simply injectedat a constant feed rate, this will cause an increase in utility costs.

[0008] More specifically, after the denitrator, an air heater, a dustcollector, a gas-gas heater (heat exchanger) and a desulfurizingabsorption tower are usually installed in that order. Yet amongst those,the chlorinating agent exerts a marked corrosive or damaging effect onthe heat exchanger used for cooling purposes. Another problem is thatsince the chlorinating agent is introduced into the desulfurizingabsorption tower, the chlorine concentration in the absorbing fluidincreases causing corrosion or damage of the metallic parts of thetower. Moreover, an increase in chlorine concentration within thedesulfurizing absorption tower may cause a reduction in oxidationcapability during desulfurization or a reduction in desulfurizationcapability itself, leading to a reduction in the overall performance ofthe system. Furthermore, an increase in chloride concentration may causean increase in the foamability of the absorbing fluid, possibilityraising the pressure loss within the absorption tower and causing anincrease in operating power.

DISCLOSURE OF THE INVENTION

[0009] In view of the above-described problems, the present inventorsmade intensive investigations for the purpose of developing a mercuryremoval method employed in an exhaust gas treatment system to removemercury (in particular, metallic mercury vapor) contained in alarge-volume gas such as exhaust gas from an electric power plant. Thismethod properly controls or regulates the amount of chlorinating agentadded to remove mercury and thereby makes it possible to achieveefficient operation of the system and maintain the performance thereofwithout exerting any adverse influence on the downstream units.

[0010] As a result, the present inventors have now found that, insteadof introducing a chlorinating agent simply on the upstream side of thedesulfurizer, the above-described problems can be solved by monitoringthe mercury concentration continuously at a position afterdesulfurization (e.g., the outlet of the desulfurizing absorption tower,the outlet of the dust collector, or the outlet of the reheater) andadding a necessary and sufficient amount of a chlorinating agentaccordingly. The present invention has been completed from this point ofview.

[0011] That is, the present invention provides a method for thetreatment of mercury present in exhaust gas wherein, after achlorinating agent is added to exhaust gas containing nitrogen oxides,sulfur oxides and mercury, the exhaust gas is subjected to a reductivedenitration treatment in the presence of a solid catalyst and then towet desulfurization using an alkaline absorbing fluid, the method beingcharacterized by measuring the mercury concentration in the exhaust gasafter the wet desulfurization; calculating a predicted value of theinlet mercury concentration before the reductive denitration treatmenton the basis of the measured mercury concentration; and controlling thefeed rate of the added chlorinating agent prior to the reductivedenitration treatment, according to the deviation of the predicted valuefrom a reference inlet mercury concentration. The mercury concentrationin the exhaust gas after the wet desulfurization may be measured at aposition prior to any of the wet dust collector, reheater or stackinstalled on the downstream side of the desulfurizer. Where a coolingtower is installed before the desulfurizing absorption tower, mercurychloride is also removed in the cooling tower. Accordingly, the mercuryconcentration may also be measured at a position prior to thedesulfurizing absorption tower installed downstream of the coolingtower. Since the inlet mercury concentration may vary according to theboiler load and the type of coal, the reference inlet mercuryconcentration is defined as a mercury concentration previously measuredat the outlet of the boiler for each type of coal or a mercuryconcentration calculated from the mercury content of coal, and refers tothe sum of the concentrations of metallic mercury and mercury chloride.

[0012] Moreover, the present invention provides a method for thetreatment of mercury present in exhaust gas wherein, after achlorinating agent is added to exhaust gas containing nitrogen oxides,sulfur oxides and mercury, the exhaust gas is subjected to a reductivedenitration treatment in the presence of a solid catalyst and then towet desulfurization using an alkaline absorbing fluid, the method beingcharacterized by measuring the mercury concentration in the exhaust gasbefore the wet desulfurization; calculating a predicted value of theoutlet mercury concentration after the wet desulfurization on the basisof the measured mercury concentration; and controlling the feed rate ofthe added chlorinating agent prior to the reductive denitrationtreatment, according to the deviation of the predicted value from areference outlet mercury concentration. The mercury concentration in theexhaust gas before the wet desulfurization may be measured at a positionprior to any of the denitrator, air heater (A/H), heat exchanger,electrostatic precipitator or desulfurizer. The reference outlet mercuryconcentration is a target value of the outlet mercury concentration.However, since a delay in response usually occurs in the control system,the reference outlet mercury concentration is defined as a valueobtained by subtracting the magnitude of concentration fluctuations fromthe upper limit of the emitted mercury concentration.

[0013] Furthermore, the present invention provides a method for thetreatment of mercury present in exhaust gas wherein, after achlorinating agent is added to exhaust gas containing nitrogen oxides,sulfur oxides and mercury, the exhaust gas is subjected to a reductivedenitration treatment in the presence of a solid catalyst and then towet desulfurization using an alkaline absorbing fluid, the method beingcharacterized by measuring the mercury concentration in the exhaust gasbefore and after the wet desulfurization; and controlling the feed rateof the added chlorinating agent prior to the reductive denitrationtreatment, according to the deviation of each of the measured mercuryconcentrations from a reference mercury concentration. According to thismethod, the accuracy of a predicted value of the mercury concentrationcan be improved by detecting the boiler load or the operating loadsignal of the electrostatic precipitator and the desulfurizer.

[0014] In addition, the present invention provides a system for thetreatment of exhaust gas wherein, after a chlorinating agent is added toexhaust gas containing nitrogen oxides, sulfur oxides and mercury bymeans of a chlorinating agent feeding device, the exhaust gas issubjected to a denitration treatment in the presence of a solid catalystof a reductive denitrator and then to desulfurization using an alkalineabsorbing fluid within a wet desulfurizer, the system beingcharacterized in that a mercury concentration meter is provided ondownstream side of the wet desulfurizer, and a flow rate control signalfrom an arithmetic unit and a controller which are connected to themercury concentration meter is sent to the chlorinating agent feedingdevice. In the aforesaid arithmetic unit, a predicted value of the inletmercury concentration before the reductive denitration treatment isprimarily calculated on the basis of the mercury concentration in theexhaust gas which is measured at a position after the wetdesulfurization and prior to the wet dust collector, reheater or stack.Then, the aforesaid controller controls the feed rate of thechlorinating agent through a signal representing the deviation of theinlet mercury concentration calculated by the aforesaid arithmetic unitfrom a predetermined reference inlet mercury concentration.

[0015] Thus, the present invention provides a mercury removal methodemployed in an exhaust gas treatment system to remove mercury (inparticular, metallic mercury vapor) contained in a large-volume gas suchas exhaust gas from an electric power plant, which controls or regulatesproperly the amount of chlorinating agent added for the removal ofmercury and thereby makes it possible to achieve efficient operation ofthe system and maintain the performance thereof without exerting anyadverse influence on the downstream units.

[0016] Specifically, with respect to the units installed on thedownstream side of the denitrator, such as the air heater, dustcollector, gas-gas heater (heat exchanger) and desulfurizing absorptiontower, the problem of corrosion or damage due to the addition of anexcess of the chlorinating agent can be prevented effectively. Moreover,since an increase in chlorine concentration within the desulfurizingabsorption tower is suppressed, a reduction in oxidation capability ordesulfurization capability during desulfurization and an increase in thefoamability of the absorbing fluid can also be prevented. Thus, theoverall performance of the system, including the desulfurizationcapability, can be maintained or improved.

[0017] Furthermore, according to the present invention, the utilitycosts required for operation can be minimized by optimizing the feedrate of the chlorinating agent (e.g., hydrogen chloride) and saving anyexcess thereof.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIG. 1 is a schematic flow diagram illustrating one example of themercury treatment method of the present invention;

[0019]FIG. 2 is a flow diagram illustrating an outline of the testingsystem used in Example 1 of the present invention;

[0020]FIG. 3 is a diagram showing the conditions employed and theresults of measurement obtained when the system of FIG. 4 was used inExample 1;

[0021]FIG. 4 is a flow diagram illustrating an example of the controlsystem used when the mercury treatment method of the present inventionis carried out;

[0022]FIG. 5 is a flow diagram illustrating another example of thecontrol system used when the mercury treatment method of the presentinvention is carried out;

[0023]FIG. 6 is a flow diagram illustrating still another example of thecontrol system used when the mercury treatment method of the presentinvention is carried out;

[0024]FIG. 7 is a diagram showing the conditions employed and theresults of measurement obtained when the system of FIG. 5 was used inExample 1; and

[0025]FIG. 8 is a diagram showing the conditions employed and theresults of measurement obtained when the system of FIG. 6 was used inExample 1.

[0026] In the foregoing figures, reference numeral 1 designates aboiler; 2, a denitrator; 3, an air heater (A/H); 4, a dust collector; 5,a heat exchanger; 6, a desulfurizing absorption tower; 7, a wet dustcollector; 8, a reheater; 9, a stack; 10, a chlorinating agent feedvalve; 11, mercury concentration measuring sites; 12, a mercury feedingdevice; 13, a chlorinating agent feeding device; 14, a continuousmercury meter; 15, an arithmetic unit; 16, a controller; and 17, a feedrate controller.

BEST MODE FOR CARRYING OUT THE INVENTION

[0027] In the exhaust gas treatment of the present invention, after achlorinating agent is added to exhaust gas containing nitrogen oxides(NOx), sulfur oxides (SOx) and mercury (Hg), the exhaust gas issubjected to a reductive denitration treatment in the presence of asolid catalyst and then to wet desulfurization using an alkalineabsorbing fluid. On the upstream side and/or downstream side of the wetdesulfurization, the mercury concentration in the exhaust gas ismeasured. On the basis of the measured mercury concentration, apredicted value of the inlet mercury concentration before the reductivedenitration treatment or a predicted value of the outlet mercuryconcentration after the desulfurization treatment is calculated. Then,the feed rate of the chlorinating agent added prior to the reductivedenitration treatment is controlled according to the deviation of thepredicted value from a reference mercury concentration.

[0028] Thus, not only can the added chlorinating agent be made to acteffectively on metallic mercury, but also the adverse influence ofexcessive chlorinating agent on the system can be avoided.

[0029] In the present invention, the feed rate of the chlorinating agentmay be controlled according to any of the following three principalmethods.

[0030] {circle over (1)} A method in which the outlet mercuryconcentration is detected and the feed rate of the chlorinating agent iscontrolled by a signal representing the deviation of a predicted valueof the inlet mercury concentration from a reference inlet concentration(first embodiment).

[0031] {circle over (2)} A method in which the mercury concentrationbefore desulfurization is detected and the feed rate of the chlorinatingagent is controlled by a signal representing the deviation of apredicted value of the outlet mercury concentration from a referenceoutlet concentration (second embodiment).

[0032] {circle over (3)} A method in which the mercury concentrationbefore desulfurization and the outlet mercury concentration are detectedand the feed rate of the chlorinating agent is controlled by both asignal representing the deviation of the mercury concentration beforedesulfurization from a reference inlet concentration, and a signalrepresenting the deviation of the outlet mercury concentration from apredicted value of the outlet mercury concentration calculated from themercury concentration before desulfurization (third embodiment).

[0033] Examples of the systems employing these methods are illustratedin FIG. 4 (first embodiment), FIG. 5 (second embodiment) and FIG. 4(third embodiment), respectively. Now, these embodiments of the mercurytreatment method of the present invention are specifically describedbelow with reference to the accompanying drawings.

[0034] First Embodiment

[0035] In the present invention, after a chlorinating agent is added toexhaust gas containing NOx, SOx and mercury, the exhaust gas issubjected to a reductive denitration treatment in the presence of asolid catalyst, and then to wet desulfurization using an alkalineabsorbing fluid. When this treatment method is carried out in adesulfurizing absorption tower, which is a unit within the system, theexhaust gas is brought into contact with an absorbing fluid such as acirculating lime slurry and thereby freed of SOx by absorption into theabsorbing fluid. For mercury contained in the exhaust gas, mercurychloride (HgCl₂) is also removed by dissolution into the aforesaidabsorbing fluid. However, metallic mercury (Hg) is not removed by theabsorbing fluid because mercury in metallic form has a very lowsolubility in water. Consequently, metallic mercury is contained in thedesulfurized exhaust gas as metallic mercury vapor and passes throughthe desulfurizing absorption tower.

[0036] Accordingly, in the present invention, a chlorinating agent isadded to the exhaust gas just prior to the denitrator. Thus, aftermetallic mercury is converted into water-soluble mercury chloride, theexhaust gas is introduced into the desulfurizing absorption tower.

[0037] In the present invention, the chlorinating agent is usually addedjust prior to the denitration treatment. Although the catalyst used inthe reductive denitrator may have a variety of forms, it generallycomprises a titania-based oxidation catalyst having a honeycombstructure or the like. Since (about 90% or more of) metallic mercury canbe oxidized on this catalyst, the chlorinating agent (e.g., hydrogenchloride) is introduced on the upstream side of the denitrator. That is,the solid catalyst within the denitrator acts not only as the originallyintended denitration catalyst, but also as a catalyst for convertingmetallic mercury into mercury chloride.

[0038]FIG. 1 is a schematic illustration of a system employing thetreatment method in accordance with this embodiment.

[0039] In the system of FIG. 1, exhaust gas leaving a denitrator 2installed downstream of a boiler 1 is passed through an air heater (A/H)3, a heat exchanger 5 for recovering thermal energy, and a dustcollector 4, and then introduced into a desulfurizing absorption tower6. No particular limitation is placed on the type of dust collector 4,provided that rough dust collection can be achieved before the exhaustgas is introduced into desulfurizing absorption tower 6. No particularlimitation is placed on the type of desulfurizing absorption tower 6,and it may comprise a two-tower type desulfurizer commonly used inexhaust gas treatment, or an absorption tower having a cooling towerinstalled upstream thereof.

[0040] In the above-described desulfurization system based on the wetprocess, a wet dust collector 7 and a reheater 8 are installeddownstream of desulfurizing absorption tower 6, and the exhaust gas ispassed through these units and discharged into the atmosphere through astack 9. In reheater 8, the combustion exhaust gas having a reducedtemperature is heated with thermal energy recovered by heat exchanger 5installed upstream of desulfurizing absorption tower 6. If the exhaustgas having a reduced temperature is discharged directly from the stack,the problem of white smoke production due to water vapor will arise.Accordingly, when combustion exhaust gas is discharged, it is commonpractice to heat the purified gas and discharge the resulting hot gas.For this reason, in the system based on the wet process, heat exchanger8 for the supply of heat is installed on the downstream side ofdesulfurizing absorption tower 6.

[0041] According to this embodiment, as shown in FIG. 1, a chlorinatingagent feeding device 13 is installed in the flow path extending fromboiler 1 to reductive denitrator 2. Moreover, this system is usuallyequipped with an ammonia injector (not shown) for injecting ammoniasupplied from an ammonia tank into exhaust gas.

[0042] The exhaust gas leaving boiler 1 is introduced into reductivedenitrator 2. In this reductive denitrator, the exhaust gas having NH₃and HCl or the like injected thereinto undergoes the reaction of NH₃with NOx. At the same time, metallic Hg is oxidized to HgCl₂ in thepresence of HCl. Thereafter, the exhaust gas is passed through airheater 3 and heat exchanger 5, freed of soot and dust in electrostaticprecipitator 4, and then introduced into wet desulfurizer 6 where bothSO₂ and HgCl₂ are removed from the exhaust gas. Although the exhaust gasleaving the reductive denitrator contains excess HCl, it is absorbedinto the alkaline aqueous solution (e.g., lime slurry) in thedesulfurizer and is not discharged from the stack.

[0043] Thus, the present invention relates to an exhaust gas treatmentprocess wherein NOx present in exhaust gas is removed in the reductivedenitrator, SO₂ is removed in the wet desulfurizer using an alkalineabsorbing fluid as the absorbent, and a chlorinating agent is added onthe upstream side of the denitrator. Even if NH₃, which is required forthe purpose of denitration, is not added on the upstream side of thereductive denitrator, it is possible to convert mercury into a chloridewith the aid of the chlorinating agent in the presence of the catalystof the reductive denitrator and thereby remove the mercury in the wetdesulfurizer.

[0044] The exhaust gas from which Hg has been removed in desulfurizingabsorption tower 6 is introduced into reheater 8, where it is heatedwith thermal energy recovered by heat exchanger 5. Thereafter, theheated exhaust gas is discharged from stack 9.

[0045] Thus, in the present invention, after a chlorinating agent isadded to exhaust gas, the exhaust gas is treated in the presence of asolid catalyst to convert metallic mercury into a water-soluble mercurycompound, and this water-soluble mercury compound in the exhaust gas isremoved in the wet desulfurization step. However, depending upon thefeed rate of the chlorinating agent, serious problems may arise in thatan excessively small amount of the chlorinating agent may cause muchmetallic mercury to remain and be discharged, or an excessively largeamount of the chlorinating agent may cause corrosion of the piping andunits or a reduction in desulfurization capability.

[0046] According to this embodiment, the mercury concentration in theexhaust gas after the wet desulfurization is measured, a predicted valueof the inlet mercury concentration at the inlet of the reductivedenitration treatment is calculated on the basis of the measured mercuryconcentration, and the feed rate of the chlorinating agent added priorto the reductive denitration treatment is controlled according to thedeviation of the predicted value from a predetermined reference inletmercury concentration.

[0047] Now, where hydrogen chloride is used as the chlorinating agent,the method for controlling the amount of hydrogen chloride added for theremoval of mercury is explained below.

[0048] The oxidation of mercury with hydrogen chloride can berepresented by the following reaction formula.

Hg+2HCl+½O ₂>HgCl₂+H₂O

[0049] Assuming that the oxidation rate of mercury to mercury chlorideis represented by r_(ox)=kp_(Hgin) ^(a)·p_(O2) ^(b)·p_(HCl) ^(c), the O₂and Hg concentrations cannot be controlled because they depend upon theboiler load, the type of carbon, and the like. However, the oxidationrate of mercury to mercury chloride can be controlled by varying thefeed rate of HCl.

[0050] For example, assuming that mercury contained in exhaust gasresulting from the combustion of coal exists in two forms, Hg⁰ andHgCl₂, the outlet mercury concentration can be expressed by thefollowing equation (1).

[0051] $\begin{matrix}{P_{ToHgout} = {P_{ToHgin} - \left\{ {{{P_{Hgin}\left( {1 - \eta_{ox}} \right)} \cdot \eta_{Hg}} + {\left( {{P_{Hgin} \cdot \eta_{ox}} + P_{HgCl2in}} \right) \cdot \eta_{HgCl2}}} \right\}}} & (1)\end{matrix}$

[0052] wherein

[0053] p_(ToHgin): Inlet partial pressure of total mercury.

[0054] p_(ToHgout): Outlet partial pressure of total mercury.

[0055] p_(Hgin): Inlet partial pressure of Hg⁰.

[0056] p_(HgCl2in): Inlet partial pressure of HgCl₂.

[0057] η_(ox): Degree of oxidation of metallic mercury.

[0058] η_(Hg): Degree of removal of metallic mercury in the exhaust gastreatment system.

[0059] η_(HgCl2): Degree of removal of mercury chloride in the exhaustgas treatment system.

[0060] In this equation, the inlet partial pressure of mercury variesaccording to the boiler load and the type of coal. Moreover, the degreeof removal of mercury in the exhaust gas treatment system is the degreeof removal of mercury in the dust collector, the desulfurizingabsorption tower and the like, and hence depends upon the operatingconditions of these units.

[0061] Generally, since the degree of removal (η_(Hg)) of metallicmercury (Hg⁰) is lower than the degree of removal (η_(HgCl2)) of mercurychloride (HgCl₂), the degree of removal of mercury in the exhaust gastreatment system can be enhanced by increasing the amount of metallicmercury oxidized to mercury chloride. Moreover, since the degree ofoxidation (η_(ox)) of metallic mercury depends upon the feed rate ofHCl, the amount of catalyst charged, the catalyst temperature, and thelike, the degree of oxidation of metallic mercury can be enhanced byincreasing the feed rate of HCl.

[0062] Accordingly, in this embodiment, the feed rate of thechlorinating agent and the degree of oxidation of metallic mercury arecontrolled by using the control system illustrated in FIG. 4.

[0063] The outlet mercury concentration A is detected at a positionselected from the desulfurizing absorption tower outlet 11 a (or thecooling tower outlet when a cooling tower is installed), the wet dustcollector outlet 11 b, and the reheater outlet 11 c. Then, in anarithmetic unit 15, a predicted value of the inlet mercury concentrationis calculated on the basis of the feed rate of the chlorinating agent(HCl) and the degree of removal of mercury in the exhaust gas treatmentsystem (or the degrees of removal of mercury in various units).According to the type of coal and the boiler load signal X, theaforesaid predicted value Y is compared with a predetermined referenceinlet mercury concentration. A controller 16 sends a signal representingthe deviation to a chlorinating agent feed valve 10 as a command Z forcontrolling the flow rate of the chlorinating agent. As a result, theflow rate of chlorinating agent 13 is properly controlled by regulatingchlorinating agent feed valve 10.

[0064] Thus, by inputting the boiler load signal X into arithmetic unit15 for predicting the inlet mercury concentration, this control systemcan accommodate variations in boiler load. Consequently, a loadfollow-up delay due to a delay in the detection of the outlet mercuryconcentration can be prevented.

[0065] Moreover, the degree of removal of each mercury compound in theexhaust gas treatment system depends upon the operating conditions ofthe electrostatic precipitator and the desulfurizer. Accordingly, byinputting signals representing the operating conditions of theelectrostatic precipitator and the desulfurizer (e.g., the electricfield strength and other conditions of the electrostatic precipitator,and the circulation flow rate and other conditions of the desulfurizer)into arithmetic unit 15, it may be possible to calculate a more accuratepredicted value of the mercury concentration and control the feed rateof the chlorinating agent thereby. Thus, the control system canaccommodate variations in the operating conditions of the electrostaticprecipitator and the desulfurizer, and can thereby control the flow ratemore properly.

[0066] The exhaust gas to be treated in the present invention maycomprise, for example, exhaust gas from the boilers of thermal electricpower plants and factories in which a fuel containing sulfur and mercury(e.g., coal or heavy oil) is burned, or exhaust gas from the heatingfurnaces of metal processing shops, petroleum refineries, andpetrochemical plants. Usually, such exhaust gas has a low NOxconcentration, contains carbon dioxide, oxygen, SOx, soot, dust andmoisture, and is discharged in large volumes.

[0067] The reductive denitration method used in the present invention isusually a method which comprises using ammonia as a reducing agent andreducing NOx present in the exhaust gas to nitrogen in the presence of asolid catalyst. The injection of ammonia is carried out in the usualmanner.

[0068] The solid catalyst used for reductive denitration in the presentinvention may comprise, for example, an oxide or sulfate of a metal suchas V, W, Mo, Ni, Co, Fe, Cr, Mn or Cu; a noble metal such as Pt, Ru, Rh,Pd or Ir; or a mixture thereof. Such a catalytic substance may besupported on a carrier selected from titania, silica, zirconia anddouble oxides thereof; zeolite; and the like.

[0069] The chlorinating agent used in the present invention is acompound which reacts with mercury present in the exhaust gas in thepresence of the aforesaid catalyst to form HgCl₂ or HgCl. Examplesthereof include hydrogen chloride (HCl), ammonium chloride, chlorine,hypochlorous acid, ammonium hypochlorite, chlorous acid, ammoniumchlorite, chloric acid, ammonium chloride, perchloric acid, and ammoniumperchlorate; and amine salts and other salts of the foregoing acids.When hydrogen chloride is added, hydrogen chloride itself may be used asa chemical agent, or an aqueous solution thereof (i.e., hydrochloricacid) may be used. No particular limitation is placed on theconcentration of hydrochloric acid. For example, usable hydrochloricacid may range from concentrated hydrochloric acid to dilutehydrochloric acid having a concentration of about 5%. As the device foradding hydrogen chloride to the exhaust gas, there may be used aconventionally known metering pump for chemical solutions. When a saltsuch as ammonium chloride is added, it is preferable to use an aqueoussolution of the salt. The chlorinating agent may be added either beforeor after the addition of ammonia to the exhaust gas.

[0070] For the wet desulfurizer a conventional unit may be used. Theabsorbing fluid used for wet desulfurization may comprise an aqueoussolution of an absorbent such as calcium carbonate, calcium oxide,calcium hydroxide, sodium carbonate or sodium hydroxide (i.e., analkaline absorbing fluid).

[0071] Generally, the content of metallic mercury in the exhaust gas isabout 10 μg/m³·N at the outlet of the boiler and several μg/m³·N at thestack. A certain type of exhaust gas to be treated (e.g., exhaust gasresulting from the combustion of coal) may already contain some hydrogenchloride, for example, at a concentration of about 10-30 ppm.Accordingly, when the chlorinating agent is added at a constant feedrate, the concentration of metallic mercury remaining after thedenitrator may vary according to the difference in the type of exhaustgas to be treated. This makes it difficult to eliminate adverse effectsdue to the addition of an excess of the chlorinating agent.

[0072] In the present invention, the mercury concentration is measuredafter the removal of mercury chloride in the desulfurizing absorptiontower, and a proper amount of the chlorinating agent is fed according tothe measured mercury concentration. Consequently, even if the type andcomposition of the exhaust gas to be treated are changed, thechlorinating agent is not fed in excess and, therefore, any adverseinfluence on the system can be avoided. Moreover, as compared with whenthe chlorinating agent is always added at a constant feed rate, theutility costs required for operation can be reduced as much as possible.For example, where the chlorinating agent is always added to exhaust gasresulting from the combustion of coal or heavy oil, for example, at aconstant feed rate to give a concentration of about 10 to 100 ppm, theoperational control of the present invention makes it possible toachieve reliable removal of mercury by varying the feed rate to betweenabout 30%-50% of the aforesaid level. This means a saving of 50-70% ofthe amount of chlorinating agent added, leading to a marked reduction incost.

[0073] Second Embodiment

[0074]FIG. 5 illustrates an example of the system employing thetreatment method in accordance with this embodiment. For an exhaust gastreatment system, this system is such that, after a chlorinating agentfeeding device is used to add a chlorinating agent to exhaust gascontaining nitrogen oxides, sulfur oxide and mercury, the exhaust gas issubjected to a reductive denitration treatment in the presence of asolid catalyst of a reductive denitrator, and then to desulfurizationusing an alkaline absorbing fluid within a wet desulfurizer. Thus, thisexhaust gas treatment system is the same as that described above inconnection with the first embodiment, but the method for controlling thefeed rate of the chlorinating agent is different.

[0075] In this embodiment, the mercury concentration beforedesulfurization B is detected at a position (on the upstream side of thedesulfurizer) selected from the denitrator inlet, the A/H inlet, theheat exchanger inlet, the dust collector inlet, and the like. Similarlyto the above-described first embodiment, a predicted value of the outletmercury concentration is calculated on the basis of the feed rate of thechlorinating agent and the degree of removal of mercury in the exhaustgas treatment system (or the degrees of removal of mercury in variousunits). Then, the flow rate of the chlorinating agent is controlled by asignal representing the deviation of the predicted value from apredetermined reference outlet mercury concentration (or a target valueof the outlet mercury concentration). The predicted value of the outletmercury concentration is calculated according to the aforesaid equation(1). When the mercury concentration measured at the outlet of the dustcollector is used, the predicted value of the outlet mercuryconcentration is calculated by subtracting the degree of removal ofmercury in dust collector 4 from a predetermined degree of removal ofmercury in the exhaust gas treatment system (η_(Hg), η_(HgCl2)).

[0076] In the system of this embodiment, a mercury concentration meteris provided at any of the aforesaid detection sites on the upstream sideof the wet desulfurizer. This mercury concentration meter is connectedto an arithmetic unit 15 and a controller 16, which sends a flow ratecontrol signal Z to a chlorinating agent feed valve. Thus, a properamount of the chlorinating agent can be added to the exhaust gas on theupstream side of the denitrator.

[0077] Moreover, by inputting the boiler load signal and the operatingsignals of the dust collector and the desulfurizer into arithmetic unit15 for predicting the outlet mercury concentration, this control systemcan accommodate variations in boiler load and variations in theoperating conditions of the dust collector and the desulfurizer.Consequently, a load follow-up delay due to a delay in the detection ofthe outlet mercury concentration can be prevented.

[0078] Third Embodiment

[0079]FIG. 6 illustrates an example of the system employing thetreatment method in accordance with this embodiment. As an exhaust gastreatment system, this system is such that, after a chlorinating agentfeeding device is used to add a chlorinating agent to exhaust gas, theexhaust gas is subjected to a reductive denitration treatment in thepresence of a solid catalyst of a reductive denitrator, and then todesulfurization using an alkaline absorbing fluid within a wetdesulfurizer. Thus, this exhaust gas treatment system is the same asthat described above in connection with the first embodiment.

[0080] In this embodiment, similarly to the above-described secondembodiment, the mercury concentration before desulfurization B isdetected at a position (on the upstream side of the desulfurizer)selected from the denitrator inlet, the A/H inlet, the heat exchangerinlet, the dust collector inlet, and the like. On the other hand, theoutlet mercury concentration A is detected at a position selected fromthe desulfurizing absorption tower outlet (or the cooling tower outletwhen a cooling tower is installed), the wet dust collector outlet, thereheater outlet, and the like.

[0081] Then, in an arithmetic unit 15, the deviation of the mercuryconcentration before desulfurization from a reference inlet mercuryconcentration predetermined on the basis of the boiler load signal andthe type of coal is obtained. Moreover, a predicted value of the outletmercury concentration is calculated on the basis of the feed rate of thechlorinating agent and the degree of removal of mercury in the exhaustgas treatment system (or the degrees of removal of mercury in variousunits), and the deviation of the predicted value from a predeterminedreference outlet mercury concentration (or a target value of the outletmercury concentration) is obtained. Using signals representing thesedeviations, a controller 16 controls the feed rate of the chlorinatingagent by means of a chlorinating agent feeding device.

[0082] Moreover, by inputting the boiler load signal and the operatingsignals of the dust collector and the desulfurizer into arithmetic unit15 for predicting the mercury concentration, this control system canaccommodate variations in boiler load and variations in the operatingconditions of the dust collector and the desulfurizer. Consequently, aload follow-up delay due to a delay in the detection of the mercuryconcentration can be prevented.

[0083] Thus, the present invention provides a mercury removal methodemployed in an exhaust gas treatment system to remove mercury (inparticular, metallic mercury vapor) contained in a large-volume gas suchas exhaust gas from an electric power plant, which method controls orregulates properly the amount of chlorinating agent added for theremoval of mercury and thereby makes it possible to achieve efficientoperation of the system and maintain the performance thereof withoutexerting any adverse influence on the downstream units.

[0084] Moreover, according to the present invention, with respect to theunits installed on the downstream side of the denitrator, such as theair heater, dust collector, gas-gas heater (heat exchanger) anddesulfurizing absorption tower, the problem of corrosion or damage dueto the addition of an excess of the chlorinating agent can be preventedeffectively. In particular, the problem of corrosion of the heatexchanger used for cooling purposes, and the problem of corrosion ordamage of the metallic parts of the desulfurizing absorption tower dueto arise in the chlorine concentration of the absorbing fluid caused bythe introduction of hydrogen chloride can be avoided. Moreover, since anincrease in chlorine concentration within the desulfurizing absorptiontower is suppressed, a reduction in oxidation capability ordesulfurization capability during desulfurization and an increase in thefoamability of the absorbing fluid can also be prevented. Thus, theoverall performance of the system, including the desulfurizationcapability, can be maintained or improved.

[0085] Furthermore, according to the present invention, the utilitycosts required for operation can be minimized by optimizing the feedrate of the chlorinating agent (e.g., hydrogen chloride) and saving anyexcess thereof.

[0086] In order to confirm the controlling effect of a chlorinatingagent added in the present invention, the following experiments werecarried out. However, this example is not to be construed to limit thescope of the invention.

EXAMPLE 1

[0087] Using a testing system illustrated in FIG. 2, an experiment wascarried out in which the feed rate of hydrogen chloride was controlledaccording to the results obtained by monitoring with a continuousmercury meter 14.

[0088] In the testing system of this example a mercury feeding device 12is provided on the upstream side of the denitration catalyst because itis necessary to vary the mercury concentration in the exhaust gas inorder to examine the proper amount of hydrogen chloride added.

[0089] As for the test conditions, the gas flow rate was 280 m³N/h(w),the SV of the denitration catalyst was 8,000 h⁻¹, the catalysttemperature was 300-360° C., the mercury concentration was 10-100μg/m³N, and the temperature of the desulfurization absorption tower was50° C.

[0090] In this example, the inlet mercury concentration in the exhaustgas was fixed at 100 μg/m³N by means of mercury feeding device 12,altered to 30 μg/m³N after the lapse of a predetermined time, and thenreturned again to 100 μg/m³N. The feed rate of hydrogen chloride 13serving as a chlorinating agent was not varied but fixed so as to give aconcentration of 100 ppm. The target value of the outlet mercuryconcentration was fixed at 15μg/m³N.

[0091] During this test period, the outlet mercury concentration wasmeasured with a continuous mercury meter 14. The results obtained whenthe system of FIG. 4 in accordance with the first embodiment was usedare shown in the graph of FIG. 3.

[0092] From these results, it was confirmed that the measured value ofthe outlet mercury concentration varied in the same manner as the inletmercury concentration increased or decreased. Consequently, in somecases, the outlet mercury concentration fell below the aforesaid targetvalue and, therefore, the feed rate of hydrogen chloride was excessive.Changes over time of the concentration of chlorinating agent to be addedso as to avoid such excessive feeding thereof are shown by a dotted linein FIG. 3. From this example, it has been made clear that the outletmercury concentration can be adjusted to the target value by detectingthe inlet mercury concentration and controlling the amount ofchlorinating agent added.

[0093] Moreover, the results obtained under the same conditions when thesystem of FIG. 5 in accordance with the second embodiment was used areshown in FIG. 7, and the results obtained under the same conditions whenthe system of FIG. 6 in accordance with the third embodiment was usedare shown in FIG. 8.

[0094] While the present invention has been described with reference toseveral embodiments and examples, it is to be understood that they aredisclosed in order to facilitate the understanding of the invention andare in no way intended to limit the scope of the invention.

1. A method for the treatment of mercury present in exhaust gas wherein,after a chlorinating agent is added to exhaust gas containing nitrogenoxides, sulfur oxides and mercury, the exhaust gas is subjected to areductive denitration treatment in the presence of a solid catalyst andthen to wet desulfurization using an alkaline absorbing fluid, themethod being characterized by measuring the mercury concentration in theexhaust gas after the wet desulfurization; calculating a predicted valueof the inlet mercury concentration before the reductive denitrationtreatment on the basis of the measured mercury concentration; andcontrolling the feed rate of the chlorinating agent added prior to thereductive denitration treatment, according to the deviation of thepredicted value from a reference inlet mercury concentration.
 2. Amethod for the treatment of mercury present in exhaust gas wherein,after a chlorinating agent is added to exhaust gas containing nitrogenoxides, sulfur oxides and mercury, the exhaust gas is subjected to areductive denitration treatment in the presence of a solid catalyst andthen to wet desulfurization using an alkaline absorbing fluid, themethod being characterized by measuring the mercury concentration in theexhaust gas before the wet desulfurization; calculating a predictedvalue of the outlet mercury concentration after the wet desulfurizationon the basis of the measured mercury concentration; and controlling thefeed rate of the chlorinating agent added prior to the reductivedenitration treatment, according to the deviation of the predicted valuefrom a reference outlet mercury concentration.
 3. A method for thetreatment of mercury present in exhaust gas wherein, after achlorinating agent is added to exhaust gas containing nitrogen oxides,sulfur oxides and mercury, the exhaust gas is subjected to a reductivedenitration treatment in the presence of a solid catalyst and then towet desulfurization using an alkaline absorbing fluid, the method beingcharacterized by measuring the mercury concentration in the exhaust gasbefore and after the wet desulfurization; and controlling the feed rateof the chlorinating agent added prior to the reductive denitrationtreatment, according to the deviation of each of the measured mercuryconcentrations from a reference mercury concentration.
 4. A system forthe treatment of exhaust gas wherein, after a chlorinating agent isadded to exhaust gas containing nitrogen oxides, sulfur oxides andmercury by means of a chlorinating agent feeding device, the exhaust gasis subjected to a denitration treatment in the presence of a solidcatalyst of a reductive denitrator and then to desulfurization using analkaline absorbing fluid within a wet desulfurizer, the system beingcharacterized in that a mercury concentration meter is provided on thedownstream side of the wet desulfurizer, and a flow rate control signalfrom an arithmetic unit and a controller connected to the mercuryconcentration meter is sent to the chlorinating agent feeding device.